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  <title>Document</title>
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  <script>
    function fn(x,y){
      x=x||0
      y=y||0
    //  return x+y
    }
    let result=fn(1,5)
    console.log(result)
    //函数没有return默认会返回undifined
    //return 后面的代码不会执行且不能换行
  </script>
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